Precision interferometer navigation system



March 18, 1969 H. w. cooPER 3,434,144 I PRECISION INTERFEROMETERNAVIGATION SYSTEM Filed Mazch 9, 1967 AZIMUTH ANGLE ANTENNA Sheet Of 3FIGJ.

ELEVATION ANGLE ANTENNA RUNWAY THRESHOLD OF RUNWAY "25 REFERENCE PHASE wVARIABLE PHASE 2s MODULATION f f'm'd mAf GENERATOR a r22 FIG 2 mAMPLITUDE f MODULATOR REFLEX r -|e KLYSTRON 20 fm ENVELOPE I f e P I T.

o ,dH) a R0, [-4 \f +Af 2 1 4 1 f-f f O 30 I? O Joi sc 3C2 g f f O' SCfmd fo-IO Hz f0 f m d qfAf at M Af=30 Hz m QR? 4 Hz 1T2 r Rc o- 23} IANTENNA TRANSMITTED ARRAY SPECTRUM AM AM SIGNAL RECEIVED I IN PLANEFIG.2A.

March 18, 1969 H. w. COOPER 3,434,144

IREGISION INTERFEROMETER NAVIGATION SYSTEM -Filed March 9, 1967 Shes a1:g of s PARABOLIC Pl LLBOX FIGS.

RECEIVER I SYMMETRY AXIS LSB

Y FIG.4.

March 18, 1969 H. w. COOPER 3,434,144

PRECISION INTERFEROMETER NAVIGATION SYSTEM Filed March 9, 1967 Sheet 3of 3 SB f +Af is \usa ANTENNA MIXER 3OMHz IF MIXER VOR VOR NO .I NO .2

I I ANGULAR POSITION INDICATION XTAL TRANSLATION OSCILLATOR PHASEDETECTOR BAND PASS INTEGRATOR AND SEARCH SWEEP FIG].

30 MHz XTAL OSCILLATOR United States Patent Claims ABSTRACT OF THEDISCLOSURE A microwave interferometer precision navigation ground systemutilizing a fixed linear array of a carrier antenna and sidebandantennas, having their maxima in the same direction, in which the anglein space with respect to the location of the ground system is encodedupon a microwave carrier as an audiofrequency modulation whose phase,with respect to a reference phase, is advanced for angles on one side ofthe reference angle and H retarded for angles on the opposite side. Acarrier frequency, amplitude modulated by the same subcarrier frequencyon which a reference phase has been frequency modulated, is radiatedfrom the carrier antenna and only one sideband is radiated from each ofthe sideband antennas.

In the copending US. patent application Ser. No. 621,818 filed Mar. 9,1967 in the name of Gerald I. Klein, owned by the assignee of thisapplication, there is disclosed and claimed a frequency translationmodulator that is preferably used herein. The modulator is adigitally-stepped ferrite phase modulator of the serrodyne type. Thelatter type modulator is well known and has been described in manypublications, including among others, The Serrodyne FrequencyTranslator, by R. C. Cummings, in the Free. IRE, vol. 45, February 1957,pages 175 to 186, inclusive.

The particular phase modulator of said copending application is amultibit latching ferrite phase shifter with a suitable digitalswitching driver which produces accurate and uniform multiple stepapproximations to the ideal repetitive sawtooth wave function forproducing the serro'dyne phase modulation.

The uniformly stepped approximation of the sawtooth wave modulated phaseshift obtained greatly suppresses the carrier and closeain sidebands andthe large number of steps which that apparatus can produce withoutincurring high losses moves the unwanted sidebands far from the carrier.That modulator is particularly effective when used in the system of thepresent invention. However, as far as the basic inventive concept isconcerned other types of frequency translation modulators could be used.

This invention relates to a precision electromagnetic navigation systemof the interferometer type in which the ground system comprises aradiating system having at least two spaced antennas radiatingsimultaneously three electromagnetic wave signals including a modulatedmain carrier, an upper side band of which is radiated from one outerantenna and a lower sideband of which is radiated from the other outerantenna. As will appear later two embodiments of the ground system areillustrated and described. In one embodiment \only two antennas are usedwith the reference carrier and one sideband being radiated from oneantenna and the carrier and the other sideband being radiated from theother antenna. Accordingly, one sideband radiation pattern is centeredon one antenna and the other sideband radiation pattern is centered onthe other antenna. The interference carrier radiation from the twoantennas forms a reference carrier radiation pattern centered on a linespaced equally "ice from each antenna and parallel to the centerlines ofthe two sideband patterns. It should be stated here that the accuracy ofthis embodiment is dependent upon the amplitude of the carrier componentbeing equal to or greater than the maximum vectorial sum of theamplitudes of the sideband components. This embodiment, althoughentirely practical and may be preferred in some installations because ofgreater simplicity does not cover as great an angle as the otherembodiment which utilizes three antennas in which the referencecomponent is transmitted only on a third antenna spaced at the midpointbetween the two sideband antennas. In the subsequent description thethree antenna system will be described first together with its operationafter which the operation of the second embodiment will readily becomeapparent. The cooperating receiver in the aircraft demodulates the threesignals and decodes the position information represented by the signals.

There are two fundamental elements in an electromagnetic navigation orguidance systernfirst, a source of radiated electromagnetic energy whichhas some characteristic which is a function of direction, and second, ameans of encoding this information on a radiated electromagnetic wavebeam and decoding it in the airborne vehicle.

Characteristics which can be used to provide this information are thefrequency, phase, polarization, and amplitude of the carrier of theelectromagnetic wave energy. Once the characteristic has been chosen toidentify this information, there are different ways of modulating thiselectromagnetic wave energy to superimpose it on the carrier. In oneclass of systems an antenna beam is mechanically scanned by rotating theentire radiating aperture, or by maintaining the antenna fixed, butoscillating or rotating 2. primary feed in the aperture. The other classof system is that in which the modulation is accomplished electronicallyby causing the radiation phase, or amplitude, of individual antennas tovary while the antenna remains fixed. The present invention relates tothe latter type of system.

Systems of the general type to which this invention relates are known asVOR, the latter being an acronym for very high frequency omnirangesystem. In these systems, the ground station component radiates anamplitudemodulated pattern with the phase of the modulation beingreferenced to north as a zero reference. Both sidebands of theamplitude-modulated main reference carrier are impressed on each of thetwo sideband antennas and the orthogonality of the antenna system inspace is used together with a quadrature phasing of the RF energy torotate an RF pattern at an audio modulation rate. In such a system,there is necessarily a one to one correspondence between the RF phaseangle and the space direction angle.

The present invention utilizes a linear array including at least twofixed spaced radiating antennas in contradistinction to conventional VORnavigation systems previously mentioned. In the present invention thespace angle information received in the aircraft is substantially thesame as that in the VCR system, but in the former system the space angleinformation is encoded upon the transmitted carrier signal as anaudio-frequency modulation by separating .the sidebands from thecarrieror by phase modulating a portion of the carrier and radiating theupper sideband from one outer antenna, radiating the lower sideband fromthe other antenna, with the carrier and a phase reference being radiatedon a separate center antenna or being equally radiated on the outerantennas so that the effective center of radiation is midway between thetwo outer antennas. The difference in path length from the groundantennas to the antenna in the aircraft results in a microwave phaseshift which is automatically translated into a shift of audio phase,which can then be measured precisely with conventional audiophase-measuring circuitry. Thus, from a precise audio measurement themicrowave phase difference between each of the elements of the groundarray is translated into a spatial angle, with respect to the referenceangle, from the center of the array to the center of the airplaneantenna. The zero reference for the phase displacement of the radiatedsidebands is carried by the frequency modulated subcarrier which in turnis amplitude modulated with the frequency used to create the coursesidebands. This reference subcarrier is demodulated in the airplanereceiver and the phase of the frequency-modulated reference is comparedwith phase of the modulation from the course (angle) information signal.

The unique feature of the present invention is that by separating thesidebands and placing one course sideband only on each of the respectiveouter radiating antennas with the carrier being divided between the twosideband antennas in the one instance and being radiated from the centerradiating antenna in the other instance, the limitation of the one toone correspondence between the RF phase angle and the space phase angleis eliminated. The phase reference is encoded by amplitude modulation ofthe carrier (or alternatively one of the course sidebands) by thefrequency modulated subcarrier. As a matter of fact, with the presentinvention the ratio of the RF phase to the space phase can be 180 orgreater as distinguished from the VCR system which is limited to unity.

From here on the description of the invention will be with reference tothe details of three antenna embodiment after which reference will bemade again to the two antenna system. As will be explained in greaterdetail hereinafter, in the present three antenna system the power at thecarrier frequency, which may be designated f is divided into threeparts. By means of suitable signal translators between the carriertransmitter and the outer respective radiating antennas only, the uppersideband, f +Af, is impressed upon the upper radiating element and thelower sideband, f ,A;f, is impressed upon the lower radiating element.Also, modulation means between the transmitter and the center radiatingantenna applies a subcarrier frequency, f which is frequency modulatedat frequency A the latter frequency being the amount of frequencytranslation added to and subtracted from the carrier and radiated fromupper and lower radiating antennas respectively.

It will be seen that the use of the three radiating element groundsub-systems of the present invention to radiate the signal containingthe angle and reference information is analogous to the conventionaldouble-sideband amplitudernodulated system, which is conventionally usedin broadcast applications. A double-sideband system is very tolerant ofany misadjustments in the receiver. Since a precise measurement of thephase shift of the sidebands with respect to the carrier is required,this tolerance to receiver characteristics is extremely valuable. It isapparent by analogy that the system could be operated as adoublesideband suppressed-carrier system or a single-sideband systemwith reduced carrier. In either of these cases the reference modulationcould be transmitted on one of the radiating elements in order to beable to make the phase measurements. The use of the three radiatingelements also allows the slightly increased gain by virtue of the thirdelement.

From the above it will be apparent that it is an object of the presentinvention to provide a novel and improved precision navigation system,in which the angular scale factor may be set as desired.

Other and further objects will become apparent from the followingdescription when taken in consideration of the accompanying drawings inwhich:

FIGURE 1 is a schematic illustration of the environment in which thepresent invention may be used;

FIGURE 2 is a schematic blQFli circuit diagram of which is referred toherein as the three antenna embodiment;

FIGURE 2A is a spectrum graph indicating the signal existing in theoperational range of the system;

FIGURE 3 is an isometric view of the three antennas of the transmittingantenna array represented in FIGURE 2;

FIGURE 4 is a graphical representation of the antenna array geometry;

FIGURE 5 is a graph showing the relative phase of the signalstransmitted from each of the antennas of the ground antenna;

FIGURE 6 is a block circuit diagram of a second embodiment, referred toas the two antenna embodiment; and

FIGURE 7 is a simplified block circuit diagram of a receiver of thepresent system.

In the example of the background environment, illustrated in FIGURE 1,in which the present invention may be used, two transmitting antennaarrays 5 and 6 are shown, the antenna array 5 being adapted to transmitelevation course angle information in an instrument landing system foraircraft. Since such systems must provide both azimuth and elevationinformation it is apparent why two antenna arrays are used. This is onlyillustrative of a situation where the present invention could be usedand since such landing systems are well known no detailed descriptionneed be given. Basically, the system for azimuth and elevationinformation are the same and operate in the same manner and thereforeonly one system will be described.

From what has been said previously it will be clear that the function ofthe ground-based sub-system in accordance with the present invention hasthe function of defining a time modulated radiation pattern such thatthe modulation existing at any point in the coverage section, whether itis azimuth or elevation, is a unique function of its angular positionwith respect to some known reference line. Subsequent processing of thismodulation in the airbrone receiver determines the desired space angleinformation.

To complete the description of the present invention, further referencewill be made only to that part of the ground-based sub-system whichincludes antenna array 5 which, in this case, is a linear array ofseparate radiating elements, the specific form shown being radiatinghorns 11, 12 and 13, coupled to their respective sources ofelectromagnetic wave energy through separate feed lines 14, 16 and 17,respectively. In this particular instance, the coupling between thefeedlines and the respective horns is through devices known in the artas parabolic pill boxes 14a, 16a and 17a. The exact form of theradiating element constitutes no part of the invention and its selectionfor use in the system is arbitrary, except insofar that it is veryessential for maximum accuracy, that the radiating elements be highlydirectional in the vertical plane to minimize ground reflections, andthat the radiation patterns of each of the antennas coincide in space.The electrical length of all lines at carrier frequency f between themicrowave generator and the centers of radiation must be equal modulo21rN radians, where N is any integer.

A common source of microwave energy for the three antenna array is themicrowave generator or transmitter 18 and the microwave energy issupplied over a microwave guide 19 to separate modulators 21, 22 and 23.A portion of the carrier power at frequency 11, is supplied directly tomodulator 22 while another portion of the carrier power is suppliedthrough a directional coupler 20 and a branch Waveguide 24 and a magic-Tcoupler 30 and branch waveguides 31 and 32 to the modulators 21 and 23,respectively. The coupler 20 must be so adjusted so that the amplitudeof the carrier component is equal to or greater than the maximumvectorial sum of the sideband components. The modulators 21 and 23 areactually frequency translators, or phase shifting devices, of identicalconstruction. These modulators are preferably of the type described insaid copending application, but may be of other suitable types. Amodulation generator 25 comprises an oscillator G which generates the 30Hz. audio frequency, A and an oscillator G which generates the 10 kHz.subcarrier frequency f,,. It is apparent from the circuit diagram ofFIGURE 2 that the audio frequency signal A}, is supplied to theoscillator G and to frequency translation modulators 21 and 23.Accordingly, the sub-carrier oscillator G is frequency modulated atfrequency, A and in turn that portion of the carrier power at frequencyi supplied to the center antenna 12 is amplitude modulated with thisfrequency modulated sub-carrier.

The modulator 21 is adjusted to add the modulation frequency A toprovide the upper sideband frequency j -l-Af, and the modulator 23 isadjusted to subtract the frequency, Af, to provide the lower sidebandfrequency, f Af. A portion of the carrier power at frequency f from thetransmitter 18 is amplitude modulated in the modulator 22 by thereference phase signal appearing on the connection 26, that is, theoutput of the frequency modulated output of the oscillator G so that theantenna 12 receives and transmits signals 12,, fmd at M and f f fmd at Aas illustrated in the graph of FIGURE 2. The subcarrier frequency fconstitutes a reference modulation for the phase displacement of theradiated sidebands radiated from the antennas 11 and 13. As will be seenlater, when this reference subcarrier is demodulated in the receiver inthe aircraft, space angle information is obtained.

The modulators 21 and 23 may be of the Fox type, or any otherappropriate type, which is capable of supplying the M frequencytranslation.

Preferably, the frequency translation (phase modulation) modulators 21and 22 are of the type described in the aforementioned copending patentapplication. The details of the amplitude modulator 22 for the centerantenna 12 are not given because this modulator may be of conventionalconstruction and conveniently may be of the semiconductor diode type.

The antenna array illustrated in FIGURE 3 has a radiation pattern inwhich the loci of constantphase of the radiated energy are cones aboutthe axis containing the radiating elements of the system. In otherwords, the radiation pattern of the antenna of FIGURE 3 has its axislying in the plane of the paper and coinciding with the axis of thecenter antenna 12.

Referring now to FIGURE 4, assuming that the above type of antenna arrayis used, the nature of the modulation pattern in space may be determinedby assuming that the phase centers of the three radiating antennas 11,12 and 13 are on a straight line YY, and that the centers of the outerradiating elements are separated by a distance L from the symmetry axisXX and center of the center antenna 12. Angular position, 0, of thereceiver at X is measured from the axis of symmetry, XX; that is, a linenormal to the line YY through the three radiation centers RC RC and RCand passing through the center radiating antenna 12. The radiationcenters RC RC and RC are the centers of radiation of the antennas 11, 12and 13, respectively. The axis of symmetry, XX, is the reference axisfor determining elevation angle, 0, assuming that the ground sub-systemis mounted for determining the elevation angle.

Although the system is not theoretically limited to any particularfrequency, the wavelength does have a bearing on the selected frequencysince the directivity that can be realized with a specified sizeradiating aperture is inversely proportional to the wavelength. Forpractical reasons a carrier frequency, f,, in the X-band, atapproximately gHz., has been selected for a practical embodiment whichhas been constructed.

Referring to the operation of the present invention, it will first beassumed that the receiver is capable of decoding the elevation angleinformation which resides in the phase characteristic of the radiationfrom the center carrier antenna 12 and the two sideband antennas 11 and13. Referring further to FIGURE 4, the lengths of the propagation pathsR and R from the centers of the upper and lower sideband antennas 11 and13, respectively, to the receiver at Z are calculated in terms of thepath length R from the center 0 of the antenna array, at theintersection of the X- and Y-axes, the spacing between the centers(centers of radiation) of the two sideband antennas 11 and 13 and theangle 0, Stated mathematically:

When the receiver is off the axis of symmetry, XX, as in FIGURE 4, itwill be apparent that R does not exactly equal R But for practicalpurposes the distance R between the transmitter, measured from RC andthe receiver at Z is very much greater than the distance L between thecenters of each of the two sideband antennas 11, 13, and therefore thelengths of paths R and R can be considered to be approximately equal,but it is this small difference that we are measuring to determine angleinformation.

For the case where R is much greater than L, the paths R and R areessentially parallel and /2 (L/R) can be neglected. If the receiver ison axis, XX, R is equal to R and is approximately equal to R. As thereceiver at Z moves above the axis of symmetry (in FIGURE 4), Rdecreases by L sin 0 while R increases by the same amount; for theangles below the axis of symmetry the opposite situation prevails.

Now consider the situation where signals are radiated from the threeradiating antennas 11, 12 and 13 of the particular antenna arrayillustrated. As previously mentioned, center antenna 12 radiates thecarrier at frequency f (10 gHz.) carrying a sub-carrier at frequency f(10 kHz.), frequency modulated at the audio frequency Af (30 Hz). Themodulator 21 is used to derive the upper course sideband, f-i-Af, whichis radiated from antenna 11 and modulator 23 is used to derive the lowercourse sideband, f Af, which is radiated from the lower sideband antenna13.

Now, assume the receiver at Z to be on the axis of symmetry XX and theinstantaneous phase of the two sidebands such that they add along thevector representing the carrier frequency T as illustrated in the vectordiagram of FIGURE 5 at A. In this vector diagram the long arrow f on theleft represents the carrier and the smaller upper and lower arrowsrepresent the upper and lower sidebands f -l-Af and f -A respectively.If the receiver moves transversely to the axis of symmetry XX, eachsideband path changes by L-sin 0, as previously described, and thecorresponding phase angle illustrated in FIGURE 5, at B and at C isgiven by:

Si 3 2 x 2. (5)

where 7\ is the wavelength corresponding to the carrier frequency f forsmall values of 0 with the understanding that the modulation frequencyis extremely small com pared to the carrier frequency. In a practicalembodiment of the present invention the carrier frequency f used wasapproximately 10 Hz., the frequency of the subcarrier f was 10 Hz., andthe audio frequency A) was 30 Hz.

In FIGURE 5 t in the graph C is used as the time reference and thedifference in angle of the vectors in A and B represent the relativephase between the reference sub-carrier frequency f and the sidebands ata given instant of time, but at different angular positions in space.

Both represent amplitude modulation of the same amount but the phase ofthe detected modulation phase of the lower sideband in B is advancedover that in A. This is for the condition where the receiver is at apoint Z, above the axis of symmetry XX, as indicated in FIGURE 4. If apoint below the axis of symmetry were considered, a similar situationwould exist, but the modulation phase would be retarded in comparisonwith that indicated at A. Thus, a fixed percentage of amplitudemodulation exists over a given region of space, but the phase of themodulation is a unique function of the angular position of the receiverwith respect to axis X-X. This is the heart of the present invention.This particular choice of antenna geometry and transmitted frequenciesexhibits the unique property of translating a given microwave phaseshift, due to a minor path length variation, directly into an identicalphase shift at the modulation frequency. When the sub-carrier, f of themicrowave carrier, f containing the 30 Hz. reference phase istransmitted from the carrier radiating antenna 12, and the 30 Hz. coursesidebands are transmitted from sideband radiating antennas 11 and 13 theairborne receiver can detect the composite signal, measure the actualmodulation phase with respect to the reference phase on the sub-carrierand thereby compute the angle 0; that is:

Since, at a given point and space angle, can be measured only over therange plus or minus 11' radians, there can be ambiguity in determinationof 0, and a multilobe pattern structure exists within the beam width ofthe individual radiating elements for radians, while the first nulls inthe pattern of the individual antenna elements are given by:

4; null :l: L

Therefore, the ambiguous angle measurement will exist over some portionof the antenna pattern, even for the most favorable conditions. Thisambiguity may easily be resolved by using a second set of sidebandantennas or a second carrier frequency and modulations to provide afine-coarse system.

From the above analysis, it will be seen that if all parts of the systemare perfect, the ability to measure the angular position, such as thatof the receiver at position Z, is limited by the ability to measure thephase of the modulation signal. One of the salient features of thepresent invention is that it operates to make one spatial degreecorrespond to many degrees of electrical phase in order to obtaingreater resolution, and the ratio is given by:

Electrical degrees q Spacial degrees 0 T A (10) In designing such asystem it is necessary to select a value of L/x such that, with theanticipated limitation in measuring phase, the required precision inthat angular position can be obtained. As a practical example ofparameters which will provide unambiguous angle coverage over a range of$0.7", a ratio of LM-40 may be selected. This will yield a ratio of 5/0equal approximately to 250 so that a position measurement can be made toan accuracy of approximately 0.01 degree.

It should also be pointed out here that the term /z(L/R) previouslyshown in Equations 3 and 4, can have significant effects at short rangessince it represents an effective phase shift between the actual receivercarrier and that which would be effected on the basis of sidebandinformation alone.

The inventive concept of the present invention has been describedprimarily in terms of a specific embodiment using a three antennatransmitting ground sub-system component. It should be apparent thatcertain departures from that specific embodiment may be made withoutdeparting from the spirit of the invention. For example, a secondembodiment utilizing only two radiating antennas is shown in FIGURE 6with corresponding components being indicated by primed referencecharacters to designate the components corresponding to similarcomponents designated by the same unprimed reference characters in thethree antenna embodiment. It is apparent that the only functualdifference between the two embodiments is that, whereas the threeantenna embodiment has three real centers of radiation, the two antennaembodiment has a virtual center of radiation for the carrier frequencycorresponding to the center antenna 12 of FIGURE 2, but the other tworeal centers of sideband radiation are the same as in the three antennaembodiment.

It is believed that the operation of the two antenna embodiment is soapparent as not to justify detailed word description. However, it shouldbe mentioned that the hybrid junction directional couplers 40 and 41should be adjusted to maintain the relation between the amplitudes ofthe sidebands and the amplitude of the carrier within the limitspreviously mentioned in order not to reduce the precision of the twoantenna embodiment.

It is apparent to one versed in the art that the fundamental requirementfor radiating a carrier in which the phase is invariant with angle isthat its center of radiation must be at the center and on the linejoining the sideband antennas. Widest angular coverage is realized by anantenna located at the center which has a radiation pattern as afunction of angles which is identical with those of the sidebandantennas. However, by connecting the sideband antennas as a cophasedarray it is apparent that its center of radiation meets the symmetryrequirements.

However, whereas in the three antenna embodiment the percentage ofmodulation of the carrier by the course sidebands is invariant withangle, in the two antenna embodiment the carrier pattern in the plane ofangle measurement is sharper by the amount of the array factor. Thus, asthe receiver in the aircraft moves off the line normal to the array(which is the direction of the maximum of the carrier) the carrierdecreases and thus the modulation percentage increases. The usablecoverage angle is the sector in which the sum of the sideband voltagevectors remains equal to or smaller than the carrier voltage vector.

It should be noted further that although the preferred embodiment willprobably be that in which the sub-carrier is modulated upon the carrierwhich is the largest signal. However, the system would work equally aswell if the reference sub-carrier were transmitted on one of thesideband antennas.

The particular receiver sub-system illustrated in FIG- URE 7 constitutesno part of this invention, per se, except insofar as some receiver mustbe provided to complete the system and to give utility to theground-based transmitter sub-system which is the subject of thisinvention. To review again, it is the function of the airborne receiversub-system to receive the microwave signal energy radiated by the threeradiating antennas 11, 12 and 13,

demodulate these signals to recover the ground referenceposition-dependent modulation, and process this data to provide positioninformation in a form suitable for recording or indication to the pilot.Consequently, the airborne receiver sub-system must provide circuits forautomatic acquisition and tracking of the transmitted signals to insureproper reception of signals as well as to provide demodulation of thereference information and course information and measure the phasedifference between the two.

Referring again to FIGURE 2 it will be seen that the upper sidebandantenna 11 transmits a spectral line at the upper sideband frequency (f+Af) while the other sideband radiating antenna 13 radiates the lowersideband frequency (f Af). On the other hand, the center antenna 12transmits the carrier f and two kHz. sidebands which are frequencymodulated at the frequency A1. The receiver sub-system must receivesimultaneously all of these signals, indicated in FIGURE 2(A), andprocess them in order to decode the position information which is codedinto these three signals at the transmitter.

In the simplified block diagram of the receiver in FIGURE 7, thereceiver includes the conventional antenna 50, from which the receivedsignals are supplied through a band-pass filter 51 to a mixer 52. Themixer 52 is in a phase-lock loop which includes an IF strip 53, a secondmixer 56 into which is fed a master reference frequency from a crystalcontrolled oscillator 57, an amplifier 58 and a third local oscillator59. The IF strip 53 is provided with suitable AGC controls. The outputof the IF strip 53 will be compared with the output of the crystalcontrolled oscillator 57 in a phase detector 56 and the resulting outputis amplified and supplied to the local oscillator 59, the output ofwhich is supplied to the mixer 52 to complete the phase-lock loop. Thisportion of the system is quite conventional. If the local oscillator 59was operating at a frequency less than the bandwidth of the phase-lockloop away from its correct frequency, then locking would be automaticwhen the desired signal is received. Since open loop control to thisdegree is not likely, the local oscillator 59 will initially be offsetto one side of the proper frequency and a search sweep in the integratorcircuits will be used to sweep the local oscillator frequency until theproper value is received, at which point the circuit will lock and thesweep will stop. This insures positive automatic tuning. The output fromthe IF strip also goes to another mixer 61 where it is translated infrequency to approximately 115 mHz. to feed the VOR receivers, fromwhich angle information is derived in conventional manner.

I claim as my invention:

1. A navigation system comprising an antenna array having a plurality ofradiating elements producing at least three centers of radiation,carrier wave generator means for supplying carrier wave energy to theelements of said array for transmitting simultaneously at least threeseparate electromagnetic wave signals, all having the same base carriercomponent, said means for supplying electromagnetic wave energy to theelements of said array including phase modulation means for supplying atleast one sideband above the frequency of said carrier on one element ofsaid array on one side of the carrier radiation center and at least onesideband below said carrier frequency on another element of said array,on the opposite side of the center of carrier radiation, means forgenerating a subcarrier, means for frequencymodulating said subcarrierin time coherence with the phase modulation of said main carrier toproduce upper and lower sidebands of said subcarrier, and means forimpressing said modulated subcarrier on said main carrier.

2. The combination as set forth in claim 1 in which said antenna arrayincludes two radiating elements.

3. The combination as set forth in claim 2 in which the upper sidebandof said carrier and the upper and lower sidebands of said subcarrier areradiated from one antenna element and the lower sideband of said carrierand the upper-and lower sideband of said subcarrier are radiated fromthe other antenna element.

4. The combination as set forth in claim 1 in which said antenna arrayincludes two outer radiating elements and an intermediate radiatingelement.

5. The combination as set forth in claim 4, in which the upper sidebandof said carrier is radiated from one of the outer antenna elements ofsaid antenna array, the lower sideband of the carrier is radiated fromthe other outer antenna element, and the upper and lower sidebands ofsaid subcarrier are radiated from said intermediate antenna element.

6. The combination as set forth in claim 1 in which said phasemodulation means for creating sidebands on said carrier are respectiveserrodyne modulators.

7. The combination as set forth in claim 6, in which said serrodynemodulators are microwave latching ferrite digital phase shifters inwhich the phase of the microwave energy is stepped through uniform stepsand returned to zero by a digital approximation to a phase variationwhich is a sawtooth function of time.

References Cited UNITED STATES PATENTS 2,430,244 1 1/1947 OBrien 343-3,048,842 8/1962 Parker et a1. 343-108 3,111,671 11/1963 Thompson343-105 X 3,287,727 11/ 1966 Earp.

3,346,860 10/1967 Earp 343--105 3,369,237 2/ 1968 Cronkhite 343-105RODNEY D. BENNETT, 111., Primary Examiner.

MALCOLM F. HUBLER, Assistant Examiner.

